In recent years, for example, a liquid crystal display device and an electroluminescent display have widely been used as displays. In these displays, transparent electrodes are used in their display element and driving circuit section. For transparent electrically conductive films for the transparent electrodes, ITO, which has a low resistance and a good transparency, is suitable. As the method for forming a thin film of ITO, there is known a method of evaporating ITO on a substrate by sputtering, vapor-deposition or ion beam deposition. Since this method needs an expensive device such as a vacuum device, costs for producing ITO thin films rise. Furthermore, the size of a thin film which can be produced is limited and large-area deposition cannot be attained since the thin film is produced in vacuum.
In order to solve the above-mentioned problems, a film-forming method based on coating with a coating solution has been investigated as another method for forming ITO thin films. This method has advantages that the size of substrates is not limited and no especial film-forming machine is required. Thus, this method is an industrially promising method.
The coating method is roughly classified into two methods, that is, a method of coating a substrate with an ITO precursor dispersed solution, and drying and firing (calcinating) the substrate to form an ITO film, and a method of dispersing crystalline or amorphous ITO into a solvent, coating a substrate with the dispersed solution and then drying the substrate to form an ITO film. In the latter, the method includes a firing step as needed, so as to make it possible to lower the resistivity or improve the mechanical strength of the film.
As the ITO precursor, there is frequently used a gel obtained by hydrolyzing a mixed solution of an indium compound and a tin compound.
The indium compound and the tin compound may be alkoxides, halides, inorganic acid salts, organic acid salts, and other compounds thereof. These compounds, together with a complexing agent if necessary, are dissolved in a suitable solvent, and then water is added to the solution so as to hydrolyze the solution, to give a gel. The gel is made of an amorphous hydroxide. In order to make this gel into an ITO thin film made of a crystalline indium/tin composite oxide, it is essential to apply and dry the gel and then subject the dried gel to a firing step at not less than a temperature of 200 to 300° C. The thus-obtained ITO generally has a large (coarse) particle size which is uneven. Thus, the ITO does not fall easily in an even film state. Problems remain in the case that the ITO is applied in particular to fields in which evenness and fineness are required. Since the hydroxide which is an ITO precursor releases water molecules in the firing step, the volume thereof decreases. This volume decrease causes strain in the thin film and the film becomes cracked. Thus, problems such that the thickness of the thin film cannot be made large are caused.
As the crystalline ITO dispersed solution whose volume decrease is small when the solution is applied and fired, there is known an ink wherein ITO fine particles prepared by a physicochemical high-temperature process are dispersed in an organic resin and a solvent. However, ITO fine particles as prepared by such a method in general make secondary aggregates; therefore, the smoothness of the surface of a thin film formed by applying and firing the ITO fine particles is unsatisfactory. This causes haze and other problems, so that the light transmissivity lowers.
As the method for preparing a crystalline metal oxide in a solution, there is known a method of hydrolyzing a metal compound and then subjecting the resultant to hydrothermal treatment in an autoclave. JP-B-5-86605 (pages 2 to 5 and Tables 1 to 4, “JP-B” means examined Japanese patent publication) discloses a method of adding an aqueous ammonium bicarbonate solution to a mixed solution of tin tetrachloride and antimony trichloride to generate a coprecipitation gel of Sb and Sn, and subjecting the gel to hydrothermal treatment, thereby preparing a crystalline tin/antimony oxide sol. In this method, all of the preparing steps are performed in aqueous solution. Even if tin tetrachloride and indium trichloride are used and attempted to prepare ITO by the same method, the resultant is a mixed hydroxide of indium and tin.
A great number of examples wherein a metal alkoxide is used as the above-mentioned metal compound are reported. Even in the case that such an alcohol/water mixed solvent is used and the alcohol content by percentage is very high, a product obtained by heating treatment using an autoclave varies dependently on the kind of the metal. In the case of using indium alkoxide and tin alkoxide, the resultant product is a mixed hydroxide of indium and tin.
Japanese Patent No. 2679008 (pages 2 to 4 and FIG. 1) discloses a method of dispersing a mixed hydroxide of indium and tin in an organic solvent in a colloidal state, subjecting the dispersed solution to azeotropic distillation, and drying/firing the resultant, thereby producing ITO powder. However, this method is different from and cannot be applied to the present invention, which is to produce a crystalline ITO dispersed solution.